Topic # 08
Structuring System Process
Requirements
CIS Life Cycle and Requirements Structuring Stage
Objectives
1. Data Flow Diagrams
2. Rules and Guidelines to DFD development that lead to accurate and
well-structured process models.
3. DFD decomposition into lower-level diagrams.
4. Balancing higher-level and lower level data flow diagrams.
5. Four main types of DFDs:
a) current physical,
b) current logical,
c) new physical, and
d) new logical.
Data Flow Diagrams
Data Flow Diagrams
 In this class, our focus will be on one tool used to coherently represent the
information gathered as part of requirements determination – Data
Flow Diagrams (DFDs).
 A Data Flow Diagram is a picture of the movement of data between external
entities and the processes and data stores within a system.
 Systems analysts use data flow diagrams to help them model the processes
internal to an information system and how data from the system’s environment
enter the system, are used by the system, and are returned to the
environment.
 DFDs help analysts understand how the organization handles information and
what its information needs are or might be.
 Analysts also use DFDs to study alternative information handling procedures
during the process of designing new information services.
An Example of Data Flow Diagram
Data Flow Diagramming Mechanics
Data Flow Diagramming Mechanics
 DFDs are versatile diagramming tools. With only four (4) simple symbols, you can
use data DFDs to represent both physical and logical information systems.
 There two different standard sets of DFD symbols, but each set consists of four
symbols that represent the same things: data flows, data stores, processes/actions,
and sources/sinks (or external entities).
 A Data Flow
can be best understand as data in motion, moving from one
place in a system to another.
 A Data Store
is data at rest (files, folders, etc.).
 A Data Process is the work or actions performed on data so that they are
transformed, stored, and/or distributed.
 A Data Source
is the origin and/or destination of data, sometimes referred to
as external entities.
Comparison of
1) DeMarco & Yourdan,
and
2) Gane & Sarson
DFD Symbol Sets
DFD Rules
1) Processes cannot have only outputs, cannot have only inputs, and must have
a verb phrase label.
2) Data can only move to a data store from a process, not from another data
store or an outside source.
3) Similarly, data can only be moved to an outside sink or to another data store
by a process.
4) Data to and from external sources and sinks can only be moved by
processes.
5) Data flows move in one direction only.
6) Both branches of a forked or a joined data flow must represent the same type
of data.
7) A data flow cannot return to the process from which it originated.
8.1.5.
To
order
…
3.1.3.
To
print
…
1. No process can have only outputs or only inputs…processes
must have both outputs and inputs.
2. Process labels should be verb phrases.
3. Process units should be numbered.
5.8.4.1.
3.1. Grades
Send a
letter
Student
4. All flows to or from a data store must move through a process.
5. Data store labels should be noun phrases.
6. Data store units should be numbered.
5.8.4.1.
Dean’s
Office
Send a
letter
Student
7. No data moves directly between external entities without going
through a process. Interactions between external entities without
intervening processes are outside the system and therefore not
represented in the DFD.
8. Source and sink labels should be noun phrases.
9. Bidirectional flow
between process
and data store is
represented by two
separate arrows.
10. Forked data
flow must refer to
exact same data
item (not different
data items) from a
common location to
multiple
destinations.
11. Joined data
flow must refer to
exact same data
item (not different
data items) from
multiple sources to
a common location.
12. Data flow
cannot go directly
from a process to
itself, must go
through
intervening
processes.
Differences between Sources/Sinks and Processes
(a) Improperly Drawn DFD
Differences between Sources/Sinks and Processes
(b) Proper Use of a Process
DFD Levels
•
Context DFD
– Overview of the organizational system
•
Level-0 DFD
– Representation of system’s major processes at highest level of
abstraction
•
Level-1 DFD
– Results from decomposition of Level 0 diagram
•
Level-n DFD
– Results from decomposition of Level n-1 diagram
Types of DFDs
Context Diagram
 The highest level DFD is called a context diagram.
 Context Diagram represents the system as a single process, with all the
related entities and the data flows in and out of the system. Since the
data stores of the system are conceptually inside the one process, no data
stores appear on a context diagram.
Level-0 Diagram, Level-1 Diagram, Level-N Diagram
 The next level diagram, called a level–0, decomposes the one process
from the context diagram into two to nine high–level processes. Each
process in a level–0 diagram can be decomposed if necessary.
 Each resulting diagram is called a level–1. Should processes in a level–1
diagram be decomposed, each resulting diagram would be called a level–
2 diagram. Each one of these processes would be decomposed on a
level–3 diagram, and so on.
Context DFD
Context
diagram shows
the system
boundaries,
external
entities that
interact with
the system,
and major
information
flows between
entities and the
system.
NOTE: only one process symbol, and
no data stores shown.
Context Diagram: An example of the University of Missouri –
St. Louis Student Registration System
• A single processing unit
• No data storage units
• Multiple data sources
(providers of data, receivers
of data)
http://www.umsl.edu/~sauterv/analysis/dfd/dfd_homework.html
Level-0 DFD
Level-0 DFD
shows the
system’s major
processes, data
flows, and data
stores at a high
level of
abstraction
(O after 1st
decimal point)
Processes are labeled 1.0, 2.0, etc. (with 0 at the end).
These will be decomposed into more primitive (lower-level) DFDs.
Level 0 Diagram: An example of the University of Missouri –
St. Louis Student Registration System
• Main (single) processing unit is divided
into sub-units (level-0 processing units)
• Use of data storage units
• Still multiple data sources
(providers of data, receivers of data) –
they are the same as at Context DFD
http://www.umsl.edu/~sauterv/analysis/dfd/dfd_homework.html
Level-1 DFD
Level-1 DFD shows
the sub-processes
of one of the
processes in the
Level-0 DFD.
This is a Level-1
DFD for Process
4.0. (1 digit after 1st
decimal point).
Processes are labeled 4.1, 4.2, etc. These can be further
decomposed in more primitive (lower-level) DFDs if necessary.
Level-n DFD
Level-n DFD
shows the subprocesses of one
of the processes
in the Level n-1
DFD.
This is a Level-2
DFD for Process
4.3. (2 digits after
1st decimal point)
Processes are labeled 4.3.1, 4.3.2, etc. If this is the lowest level
of the hierarchy, it is called a primitive DFD.
Multi-Level DFD: Webster-Plus system example
Context DFD
(a fragment)
Main processing unit:
to register for courses
1.0
Level-0 DFD
(a fragment)
2.1
Level-1 DFD
(a fragment)
Level-2 DFD
(a fragment)
2.0
…
2.2
2.1.2
…
2.1.9
- to add a course
- to pay for a course
…
- to display schedule
2.9
…
2.1.1
9.0
- to pay with credit card
- to pay with cache
…
- to pay with transfer of
money
- to enter CC number
- to enter FIrstName and
LastName
…
- to confirm amount to be
paid
Decomposition and Balancing of DFDs
Decomposition of DFDs
 Decomposition
is the iterative process by which a system description is
broken down into finer and finer detail, creating a set of
diagrams in which one process on a given diagram is
explained in greater detail on a lower–level diagram.
 Balancing
is the conservation of inputs and outputs to a data flow
diagram process when that process is decomposed to a
lower level. You can determine if a set of DFDs are
balanced or not by observing whether or not a process
which appears in a level–n diagram has the same inputs
and outputs when decomposed for a lower-level
diagram.
Topic # 08
Structuring System Process
Requirements:
In-Classroom Exercise
Four Different Types of DFDs
Types of DFDs
 There are actually four different types of DFDs used in system’s development process:
1) Current physical,
2) Current logical,
3) New logical, and
4) New physical.
 Current physical DFDs often show the people and technology used within a system,
showing who does what and the media on which data are
stored.
 Current logical DFDs
attempt to show the essence of the system without regard
to the actual physical implementation.
Note: Detailed DFDs for the current physical system tend to take a great deal of
time and then tend to be tossed aside as the focus shifts from the current to
the new system.
By not drawing current physical DFDs, or by drawing only cursory ones,
analysts can save themselves effort and focus from the beginning on what
is really important, the new system.
An Example of Current Physical Inventory Control
(a) Context Diagram
An Example of Current Physical Inventory Control System
(b) Level-0 Data Flow Diagram
Four Different Types of DFDs
 A current logical data flow diagram
is a model of the current information
system in which the physical aspects of the system are
removed as much as possible so that the current system
is reduced to its essence, to the data and the processes
that transform them, regardless of actual physical form.
 A new logical data flow diagram
is a model of the target information system
that depicts the functionality and data flows of the target
system without regards to how the system will be
physically implemented.
Level-0 Data Flow Diagram for Current Logical
Inventory Control System
Level-0 Flow Diagram for New Logical Inventory
Control System
Guidelines for Drawing DFDs
Additional DFD Guidelines:
 DFD Completeness
the extent to which all necessary components of a data flow
diagram have been included and fully described.
 DFD Consistency
the extent to which information contained on one level of a set
of nested DFDs is also included on other levels.
 Timing
 Iterative Development
 Primitive DFDs
the lowest level of decomposition for a DFD.
Topic # 08
Structuring System Process
Requirements:
Homework Assignment